Absorbent structures of chemically treated cellulose fibers

ABSTRACT

Disclosed are absorbent structures including fibers bound with a polyvalent cation-containing compound and superabsorbent polymer particles. The fibers exhibit an ion extraction factor of at least 5%. Also disclosed are multi-strata absorbent structures, such as disposable absorbent articles, including the treated fibers and SAP particles. Further disclosed are methods for preparing absorbent structures including the treated fibers; structures including fibers combined with a polyvalent cation-containing compound; and methods for treating or coating SAP particles with polyvalent cation-containing compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation application of U.S. patentapplication Ser. No. 10/360,147, filed Feb. 7, 2003, which is adivisional application of U.S. patent application Ser. No. 09/469,930,filed Dec. 21, 1999, which has issued into U.S. Pat. No. 6,562,743,which claims priority under 35 U.S.C. § 119, based on U.S. ProvisionalApplication Ser. No. 60/117,565, filed Jan. 27, 1999, and ProvisionalApplication Ser. No. 60/113,849, filed Dec. 24, 1998, the entiredisclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a fiber treated to enhancepermeability of an absorbent structure prepared from such fibers. Moreparticularly, the invention relates to fibers treated with polyvalentmetal ion-containing compounds for use in absorbent structures made withsuch fibers, and absorbent articles containing such absorbentstructures.

BACKGROUND OF THE INVENTION

[0003] Absorbent structures are important in a wide range of disposableabsorbent articles including infant diapers, adult incontinenceproducts, sanitary napkins and other feminine hygiene products and thelike. These and other absorbent articles are generally provided with anabsorbent core to receive and retain body liquids. The absorbent core isusually sandwiched between a liquid pervious topsheet, whose function isto allow the passage of fluid to the core and a liquid imperviousbacksheet whose function is to contain the fluid and to prevent it frompassing through the absorbent article to the garment of the wearer ofthe absorbent article.

[0004] An absorbent core for diapers, adult incontinence pads andfeminine hygiene articles frequently includes fibrous batts or websconstructed of defiberized, loose, fluffed, hydrophilic, cellulosicfibers. Such fibrous batts form a matrix capable of absorbing andretaining some liquid. However, their ability to do so is limited. Thus,superabsorbent polymer (“SAP”) particles, granules, flakes or fibers(collectively “particles”), capable of absorbing many times their weightof liquid, are often included in the absorbent core to increase theabsorbent capacity of the core, without having to substantially increasethe bulkiness of the core. In an absorbent core containing matrix fibersand SAP particles, the fibers physically separate the SAP particles,provide structural integrity for the absorbent core, and provide avenuesfor the passage of fluid through the core.

[0005] Absorbent cores containing SAP particles have been successful,and in recent years, market demand has increased for thinner, moreabsorbent and more comfortable absorbent articles. Such an article maybe obtained by increasing the proportion of SAP particles to thecellulose or other matrix fibers in the absorbent core.

[0006] However, there are practical limits to increasing the proportionof SAP particles in the absorbent core. If the concentration of SAPparticles in an absorbent core is too high, gel blocking can result.When SAP particles distributed through an absorbent core of matrixfibers are exposed to liquid they swell as they absorb the liquid,forming a gel. As adjacent SAP particles swell, they form a barrier tofree liquid not immediately absorbed by the SAP particles. As a result,access by the liquid to unexposed SAP particles may be blocked by theswollen (gelled) SAP particles. When gel blocking occurs, liquidpooling, as opposed to absorption, takes place in the core. As a result,large portions of the core remain unused, and failure (leaking) of theabsorbent core can occur. Gel blocking caused by high concentrations ofSAP particles results in reduced core permeability, or fluid flow, underpressures encountered during use of the absorbent product.

[0007] One way to minimize gel block (and maintain core permeability) isto limit the proportion of SAP particles to matrix fibers in theabsorbent core. In this way, there is sufficient separation betweenparticles, such that even after the particles have been swollen byexposure to liquid they do not contact adjacent particles and freeliquid can migrate to unexposed SAP particles. Unfortunately, limitingthe concentration of SAP particles in the absorbent core also limits theextent to which the core can be made thinner and more comfortable. Toavoid gel block, commercial absorbent cores are presently limited to SAPparticle concentrations of 20% to 50% by weight of the core.

[0008] It would be highly desirable to provide an absorbent core capableof bearing a SAP particle concentration exceeding 50% by weight,preferably 50% to 80% by weight, while maintaining core permeability andavoiding the problem of gel block. It would also be desirable to providean absorbent core, which exhibits improved permeability for a given SAPconcentration. At the same time, it is important to be able to blend thematrix fiber and SAP particles into an absorbent core using conventionalmaterial shipping and handling processes to provide attractive economicsfor the manufacture of infant diapers, feminine hygiene pads, adultincontinence pads, and the like.

[0009] Other methods for increasing SAP particle concentrations whileminimizing gel block, have been directed to modifying the superabsorbentpolymer itself. Modification of the superabsorbent polymer usuallyinvolves reducing the gel volume of the superabsorbent polymer particlesby increasing the crosslinking of the polymer. A crosslinked SAPparticle is restricted in its ability to swell, and therefore has areduced capacity, or gel volume. Although modified SAP particles areless susceptible to gel block, they also absorb less liquid by weightdue to their reduced gel volume. Modified SAP particles also tend to bebrittle and fracture and crack during or after processing into the finalabsorbent product. A variety of crosslinkers are known in the art. It isalso known to use polyvalent metal ions, including aluminum, during themanufacture of SAPs, to serve as an ionic crosslinking agent. See forexample, U.S. Pat. No. 5,736,595.

[0010] Crosslinking of SAP particles affects the permeability of theparticle, i.e., the ability of liquid to permeate the particle to thecenter, thereby fully utilizing the capacity of the SAP particle. Asused in this specification, SAP particle permeability is distinguishedfrom the permeability of the “core” or absorbent structure. Corepermeability refers to the ability of liquid to permeate through anabsorbent structure containing SAP particles. As used herein, suchpermeability is measured by methods including “vertical” permeabilityand “inclined” permeability. A core “permeability factor” may bedetermined from both vertical and inclined permeability measurements.

[0011] A method for improved utilization of the superabsorber isdisclosed in U.S. Pat. No. 5,147,343, where particle size distributionof the granules is controlled. By controlling the particle size of thesuperabsorber and hence the surface area, the rate of fluid uptake canbe optimized to the core design. However, the utilization of theabsorbent core is reduced at higher concentrations of SAP particles dueto gel blocking.

SUMMARY OF THE INVENTION

[0012] The present invention is directed to absorbent structuresincluding fibers bound with a polyvalent cation-containing compound andsuperabsorbent polymer particles. The fibers exhibit an ion extractionfactor of at least 5%. The present invention is also directed tomulti-strata absorbent structures, such as disposable absorbentarticles, including the treated fibers and SAP particles.

[0013] The present invention is also directed to methods for preparingabsorbent structures including the treated fibers; structures includingfibers combined with a polyvalent cation-containing compound; andmethods for treating or coating SAP particles with polyvalentcation-containing compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a perspective view of an inclined permeability testapparatus employed in the Examples of the present specification.

[0015]FIG. 2 is a graph illustrating the inclined permeability ofabsorbent structures of the present invention compared with conventionalstructures.

[0016]FIG. 3 is a perspective view of a vertical permeability testapparatus employed in the Examples of the present specification.

[0017]FIG. 4 is a graph illustrating vertical permeability ofSAP-containing absorbent structures after application of 0.9% salinesolution having various compounds dissolved in the saline at differentconcentrations.

[0018]FIG. 5 is a graph illustrating vertical permeability ofSAP-containing absorbent structures made with fibers treated withvarious compounds, or absorbent structures having various compoundsapplied to thereto.

[0019]FIG. 6 is a graph illustrating the relationship betweenpermeability factor and ion removal, for absorbent structures preparedaccording to the present invention.

[0020]FIG. 7 is a graph illustrating the relationship betweenpermeability factor and disposable diaper performance as measured byfluid wicked to diaper extremity, for absorbent structures preparedaccording to the present invention.

[0021]FIG. 8 is a graph illustrating the relationship betweenpermeability factor and absorbent structure performance, as measured byfluid wicked to structure extremity, for absorbent structures preparedaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] All patents, patent applications, and publications cited in thisspecification are hereby incorporated by reference in their entirety. Incase of conflict in terminology, the present disclosure controls.

[0023] It has now been surprisingly and unexpectedly discovered that bytreating fibers with a polyvalent ion-containing compound, an absorbentstructure (or core) made from such fibers and SAP particles exhibitsreduced gel blocking and increased core permeability. As a result, theconcentration of SAP particles in an absorbent core may be increasedwithout experiencing gel block or loss in permeability of the core. Thisallows for better utilization of the absorbent core, because a highfluid flow can be maintained under usage pressure in the absorbent core,thus enabling manufacturers to produce thinner, more absorbent and morecomfortable absorbent structures.

[0024]FIG. 8 exemplifies the improvement in absorbent cores as thepermeability is increased. In the figure, the fluid wicked to the coreextremity refers to the last three inches of the core material asmeasured by the horizontal wicking test as described in the proceduressection. For two types of SAP, an improvement in core utilization isnoted. Further, FIG. 7 shows that for machine-made diapers, thepermeability improvement also provides an improvement in coreutilization as measured by the horizontal wicking test.

[0025] When an absorbent core made with SAP particles and fibers treatedwith a polyvalent metal-ion containing compound according to the presentinvention is exposed to liquid, the polyvalent metal ion is releasedfrom the fibers, carried by the liquid and contacts the surface of theSAP particle. The polyvalent metal ion inhibits the rate of swelling ofthe SAP particle sufficiently to enable liquid to permeate beyond theswelling SAP particles to contact unexposed SAP particles. Although therate of swelling is reduced, the extent of swelling of the SAP particlesis not significantly affected by contact with liquid containing thepolyvalent metal ion.

[0026] To prepare fibers suitable for use in an absorbent core, anycompatible polyvalent metal ion-containing compound may be employed,provided that the compound releases the polyvalent metal ion uponexposure of the treated fiber to the liquid encountered in the core. Thedegree to which the polyvalent ion is released from the fiber uponexposure to liquid is referred to herein as “ion extraction”. The degreeof “ion extraction” is related to the permeability of cores asillustrated in FIG. 6. In this figure increasing ion extraction providesincreased permeability.

[0027] It is not necessary that the compound chemically bond with thefibers, although it is preferred that the compound remain associated inclose proximity with the fibers, by coating, adhering, precipitation, orany other mechanism such that it is not dislodged from the fibers duringnormal handling of the fibers, absorbent core or absorbent articlebefore contact with liquid. For convenience, the association between thefiber and the compound discussed above may be referred to as the “bond,”and the compound may be said to be bound to the fiber.

[0028] This concept is exemplified as follows: sheeted cellulosic fiberstreated with a water insoluble aluminum compound had the same aluminumconcentration before and after hammer mill disintegration (Kamas mill).Sheeted cellulosic fibers treated with a water soluble aluminum compoundthe same aluminum concentration before disintegration (Kamas mill) andafter disintegration. Sheeted cellulosic fibers treated with a waterinsoluble and a water soluble aluminum compound had the same aluminumconcentration before disintegration (Kamas mill) and afterdisintegration.

[0029] Any polyvalent metal salt including transition metal salts may beused, provided that the compound is capable of releasing the polyvalentmetal ion upon contact with liquid encountered in the absorbent core.The polyvalent metal containing compound selected for this applicationshould be compatible with safe contact with human skin and mucousmembranes. Examples of suitable polyvalent metals include beryllium,magnesium, calcium, strontium, barium, titanium, zirconium, vanadium,chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, copper,zinc, aluminum and tin. Preferred ions include aluminum, iron and tin.The preferred metal ions have oxidation states of +3 or +4. The mostpreferred ion is aluminum. Any salt containing the polyvalent metal ionmay be employed, provided that the compound is capable of releasing thepolyvalent metal ion upon contact with liquid encountered in theabsorbent core. Examples of suitable inorganic salts of the above metalsinclude chlorides, nitrates, sulfates, borates, bromides, iodides,fluorides, nitrides, perchlorates, phosphates, hydroxides, sulfides,carbonates, bicarbonates, oxides, alkoxides phenoxides, phosphites, andhypophosphites. Examples of suitable organic salts of the above metalsinclude formates, acetates, butyrates, hexanoates, adipates, citrates,lactates, oxalates, propionates, salicylates, glycinates, tartrates,glycolates, sulfonates, phosphonates, glutamates, octanoates, benzoates,gluconates, maleates, succinates, and4,5-dihydroxy-benzene-1,3-disulfonates. In addition to the polyvalentmetal salts, other compounds such as complexes of the above saltsinclude amines, ethylenediaminetetra-acetic acid (EDTA),diethylenetriaminepenta-acetic acid (DTPA), nitrilotri-acetic acid(NTA), 2,4-pentanedione, and ammonia.

[0030] It has been surprisingly discovered that trivalent aluminum ionsare the preferred polyvalent metal ions for minimizing gel block. FIG. 4shows the effect of a variety of polyvalent metal containing compoundson vertical permeability of test cores containing SAP and cellulosefiber. This data indicates that several polyvalent metal cations producea higher vertical permeability in the test core than the aluminum salts,when the polyvalent metal containing compounds are dissolved in themobile phase (0.9% saline) of the vertical permeability test. FIG. 5shows the effect of a variety of polyvalent metal containing compoundson the vertical permeability test cores containing SAP and cellulosefiber pretreated with the polyvalent metal salt, or test cores that area mixture of SAP and cellulose fiber and the polyvalent metal salt. Thisdata indicates that the test cores containing the aluminum salts havesuperior vertical permeability to those containing other polyvalentmetal containing compounds. Accordingly, preferred compounds are thosewhich contain aluminum and are capable of releasing aluminum ions uponcontact with liquid encountered in the absorbent core. Examples of suchcompounds include aluminum salts such as aluminum chloride, aluminumsulfate and aluminum hydroxide.

[0031] Depending on the polyvalent metal ion containing compound used totreat the fiber, it may be necessary to provide other components, tocause or enhance ionization when liquid contacts the treated fiber. Forexample, if aluminum hydroxide is employed as the metal ion containingcompound, and is precipitated onto the hydrophilic fibers, it isnecessary to also treat the fiber with an ionizable acid, for examplecitric acid. When the treated fiber is exposed to liquid, such as urinefor example, the liquid will solubilize the acid, reducing the pH of theliquid and thereby ionizing the aluminum hydroxide to provide aluminumions in the form of aluminum citrate. A variety of suitable acids may beemployed, although the acid preferably should have a low volatility, behighly soluble in water, and bond to the fiber. Examples includeinorganic acids such as sodium bisulfate and organic acids such asformic, acetic, aspartic, propionic, butyric, hexanoic, benzoic,gluconic, oxalic, malonic, succinic, glutaric, tartaric, maleic, malic,phthallic, sulfonic, phosphonic, salicylic, glycolic, citric,butanetetracarboxylic acid (BTCA), octanoic, polyacrylic, polysulfonic,polymaleic, and lignosulfonic acids, as well ashydrolyzed-polyacrylamide and CMC (carboxymethylcellulose). Among thecarboxylic acids, acids with two carboxyl groups are preferred, andacids with three carboxyl groups are more preferred. Of these acids,citric acid is most preferred.

[0032] In general, the amount of acid employed is dictated by theacidity and the molecular weight of that acid. Generally it is foundthat an acceptable range of acid application is 0.5%-10% by weight ofthe fibers. As used herein, the “percent by weight,” refers to theweight percent of dry fiber treated with the polyvalent metal containingcompound. For citric acid the preferred range of application is 0.5%-3%by weight of the fibers.

[0033] As discussed above, the treatment of fibers with a polyvalention-containing compound increases core permeability. Such treatmentresults in stiffening of the fibers. The stiffened fibers do not swellin water to the extent that untreated fibers do. Consequently existinginterfiber channels or other avenues for liquid to flow through anabsorbent structure formed from the fibers are kept open to a greaterextent by the stiffened fibers than by the untreated fibers. Thereduction in wet swell that is produced by polyvalent ion treatment ofthe fibers, represents an important contribution to the overall improvedpermeability of an absorbent core containing SAP particles and thetreated fibers of the present invention.

[0034] Water retention value (WRV) is an indication of a fiber's abilityto retain water under a given amount of pressure. Cellulose fibers thatare soaked in water swell moderately, and physically retain water in theswollen fiber walls. When an aqueous fiber slurry is centrifuged, themajority of the water is removed from the fibers. However, a quantity ofwater is retained by the fiber even after centrifugation, and thisquantity of water is expressed as a percentage based on the dry weightof the fiber. All of the fibers treated according to the presentinvention, have lower WRV values than corresponding untreated fibers.Consequently, all the treated fibers are stiffer than conventional flufffibers, thus contribute to improved core permeability.

[0035] Reducing Agents

[0036] If desired, reducing agents may be applied to the treated fibersto maintain desired levels of fiber brightness, by reducing brightnessreversion. Addition of acidic substances may cause browning of fiberswhen heated during processing of webs containing the fibers. Reducingagents counter the browning of the fibers. The reducing agent shouldalso bond to the fibers. Preferred agents are sodium hypophosphite andsodium bisulfite, and mixtures thereof.

[0037] Fibers

[0038] A wide variety of fiber types may be treated with the polyvalentmetal ion containing compound. However, the use of hydrophilic fibers ispreferred. Suitable hydrophilic fibers for use in the present inventioninclude cellulosic fibers, modified crosslinked cellulose fibers, rayon,polyester fibers, hydrophilic nylon, silk wool and the like. Suitablehydrophilic fibers can also be obtained by hydrophilizing hydrophobicfibers. Fibers may be hydrophilized by treatment with surfactants,silica, or surface oxidation, e.g. by ozone in a corona discharge. Suchfibers may be derived from, for example, polyolefins such aspolyethylene or polypropylene, polyacrylics, polyamides, polystyrenes,polyurethanes and the like.

[0039] For absorbent product applications, the preferred fiber iscellulose. Examples of suitable sources of cellulose fibers includesoftwood cellulose, hardwood cellulose, cotton, esparto grass, bagasse,hemp, flax, chemically modified cellulose and cellulose acetate. Thepreferred wood cellulose is bleached cellulose. The final purity of thepreferred cellulose fiber of the present invention may range from atleast 80% alpha to 98% alpha cellulose, although purity of greater than95% alpha is preferred, and purity of 96.5% alpha cellulose, is mostpreferred. As used herein, the term “purity” is measured by thepercentage of alpha cellulose present. This is a conventionalmeasurement in the dissolving pulp industry. Methods for the productionof cellulose fiber of various purities typically used in the pulp andpaper industry are known in the art.

[0040] Curl is defined as a fractional shortening of the fiber due tokinks, twists and/or bends in the fiber. The percent curl of thecellulose fibers of the present invention is preferably from 25% to 80%,and is more preferably 75%. For the purpose of this disclosure, fibercurl may be measured in terms of a two dimensional field. The fiber curlis determined by viewing the fiber in a two dimensional plane, measuringthe projected length of the fiber as the longest dimension of arectangle encompassing the fiber, L (rectangle), and the actual lengthof the fiber L (actual), and then calculating the fiber curl factor fromthe following equation:

Curl Factor=L (actual)/L(rectangle)−1

[0041] A fiber curl index image analysis method is used to make thismeasurement and is described in U.S. Pat. No. 5,190,563. Fiber curl maybe imparted by mercerization. Methods for the mercerization of cellulosetypically used in the pulp and paper industry are known in the art.

[0042] The preferred water retention value (WRV) of the cellulose fibersof the present invention is less than 85 %, and more preferably between30% and 80%, and most preferably 40%. The WRV refers to the amount ofwater calculated on a dry fiber basis, that remains absorbed by a sampleof fibers that has been soaked and then centrifuged to remove interfiberwater. The amount of water a fiber can absorb is dependent upon itsability to swell on saturation. A lower number indicates internalcross-linking has taken place. U.S. Pat. No. 5,190,563 describes amethod for measuring WRV.

[0043] Another source of hydrophilic fibers for use in the presentinvention, especially for absorbent members providing both fluidacquisition and distribution properties, is chemically stiffenedcellulose fibers. As used herein, the term “chemically stiffenedcellulose fibers” means cellulose fibers that have been treated toincrease the stiffness of the fibers under both dry and wet aqueousconditions. In the most preferred stiffened fibers, chemical processingincludes intrafiber crosslinking with crosslinking agents while suchfibers are in a relatively dehydrated, defibrated (i.e.,individualized), twisted, curled condition. These fibers are reported tohave curl values greater than 70% and WRV values less than 60%. Fibersstiffened by crosslink bonds in individualized form are disclosed, forexample U.S. Pat. No. 5,217,445 issued Jun. 8, 1993, and U.S. Pat. No.3,224,926 issued Dec. 21, 1965.

[0044] Saps

[0045] The term “superabsorbent polymer particle” or “SAP” particle isintended to include any particulate form of superabsorbent polymer,including irregular granules, spherical particles (beads), powder,flakes, staple fibers and other elongated particles. “SAP” refers to anormally water-soluble polymer which has been cross-linked to render itsubstantially water insoluble, but capable of absorbing at least ten,and preferably at least fifteen, times its weight of a physiologicalsaline solution. Numerous examples of superabsorbers and their methodsof preparation may be found for example in U.S. Pat. Nos. 4,102,340;4,467,012; 4,950,264; 5,147,343; 5,328,935; 5,338,766; 5,372,766;5,849,816; 5,859,077; and U.S. Pat. Re. 32, 649.

[0046] SAPs generally fall into three classes, namely starch graftcopolymers, cross-linked carboxymethylcellulose derivatives and modifiedhydrophilic polyacrylates. Non-limiting examples of such absorbentpolymers are hydrolyzed starch-acrylate graft co-polymer, saponifiedacrylic acid ester-vinyl co-polymer, neutralized cross-linkedpolyacrylic acid, cross-linked polyacrylate salt, and carboxylatedcellulose. The preferred SAPs, upon absorbing fluids, form hydrogels.

[0047] Suitable SAPs yield high gel volumes or high gel strength asmeasured by the shear modulus of the hydrogel. Such preferred SAPscontain relatively low levels of polymeric materials that can beextracted by contact with synthetic urine (so-called “extractables”).SAPs are well known and are commercially available from several sources.One example is a starch graft polyacrylate hydrogel marketed under thename IM1000™ (Hoechst-Celanese, Portsmouth, Va.). Other commerciallyavailable superabsorbers are marketed under the trademark SANWET™ (SanyoKasei Kogyo Kabushiki, Japan), SUMIKA GEL™ (Sumitomo Kagaku KabushikiHaishi, Japan), FAVOR™ (Stockhausen, Garyville, La.) and the ASAP™series (Chemdal, Aberdeen, Miss).

[0048] Suitable SAP particles for use in the present invention includethose discussed above, and others, provided that the SAP particleprovides improved permeability of an absorbent core made with the SAPand a hydrophilic fiber treated according to the present invention. Mostpreferred for use with the present invention are polyacrylate-basedSAPs.

[0049] As used in the present invention, SAP particles of any size orshape suitable for use in an absorbent core may be employed.

[0050] Absorbent Core Structures

[0051] The treated fibers of the present invention may be used incombination with SAP particles, to form a stratum of an absorbent core,useful in forming an absorbent structure for use in manufacturing anabsorbent article. The treated fibers begin to show improved corepermeability in a mixture of 20% SAP and 80% fiber in an absorbent core,even better permeability is displayed in a mixture of 40% SAP and 60%fiber in an absorbent core, and further improvement in permeability isobserved in a mixture of 60% to 80% SAP and 40% to 20% fiber in anabsorbent core. Preferably, the treated fibers will be used to form onestratum of a multi-strata absorbent structure. Absorbent structuresparticularly useful in infant diapers and adult incontinence productsoften include at least two defined strata—an upper acquisition stratumand a lower storage stratum. Sometimes, a distribution stratum isprovided between the acquisition and storage strata. Optionally, awicking stratum is provided below the storage stratum.

[0052] Typically SAP particles are provided in the storage stratum,although such SAP particles may also, or alternatively be provided in adistribution stratum. The treated fibers or other treated substrates ofthe present invention may be located in any stratum, provided that uponexposure of the absorbent structure to a liquid insult, the liquidcontacts the treated fiber, and then carries the polyvalent metal ion tothe SAP particles. Preferably, in a multi-strata absorbent structure,the treated fiber of the present invention will be provided in thestorage layer.

[0053] Absorbent Articles

[0054] The treated fibers of the present invention may be employed inany disposable absorbent article intended to absorb and contain bodyexudates, and which are generally placed or retained in proximity withthe body of the wearer. Disposable absorbent articles include infantdiapers, adult incontinence products, training pants, sanitary napkinsand other feminine hygiene products.

[0055] A conventional disposable infant diaper generally includes afront waistband area, a rear waistband area and a crotch region therebetween. The structure of the diaper generally includes a liquidpervious topsheet, a liquid impervious backsheet, an absorbentstructure, elastic members, and securing tabs. Representative disposablediaper designs may be found, for example in U.S. Pat. No. 4,935,022 andU.S. Pat. No. 5,149,335. U.S. Pat. No. 5,961,505 includes representativedesigns for feminine hygiene pads.

[0056] The absorbent structure incorporating the treated fibers of thepresent invention may be formed in place by blending individualizedfibers and SAP particles and applying them to a form under appliedvacuum to create an absorbent structure of desired shape. Alternatively,the absorbent structure may be formed separately as a continuous rollgood, preferably using airlaid (or “dryformed”) technology.

[0057] Fiber Treatment

[0058] The fibers suitable for use in absorbent structures may betreated in a variety of ways to provide the polyvalent metalion-containing compound in close association with the fibers. Apreferred method is to introduce the compound in solution with thefibers in slurry form and cause the compound to precipitate onto thesurface of the fibers. Alternatively, the fibers may be sprayed with thecompound in aqueous or non-aqueous solution or suspension. The fibersmay be treated while in an individualized state, or in the form of aweb. For example, the compound may be applied directly onto the fibersin powder or other physical form. Whatever method is used, however, itis preferred that the compound remain bound to the fibers, such that thecompound is not dislodged during normal physical handling of the fiberin forming the absorbent structure and absorbent articles or use of thearticle, before contact of the fiber with liquid. Upon contact of thetreated fibers with liquid, the applied compound should be released fromthe fiber to provide ions within the liquid.

[0059] Preferred Method of Treating Fibers

[0060] In a preferred embodiment, the treated fibers of the presentinvention are made from cellulose fiber, obtained from BuckeyeTechnologies Inc. (Memphis, Tennessee). The pulp is slurried, the pH isadjusted to about 4.0, and aluminum sulfate (Al₂(SO₄)₃) in aqueoussolution is added to the slurry. The slurry is stirred and theconsistency reduced. Under agitation, the pH of the slurry is increasedto approximately 5.7. The fibers are then formed into a web or sheet,dried, and sprayed with a solution of citric acid at a loading of 2.5weight % of the fibers. The web is then packaged and shipped to endusers for further processing, including fiberization to formindividualized fibers useful in the manufacture of absorbent products.If a reducing agent is to be applied, preferably it is applied before adrying step and following any other application steps. The reducingagent may be applied by spraying, painting or foaming.

[0061] Without intending to be bound by theory, it is believed that bythis process, the soluble Al₂(SO₄)₃ introduced to the pulp slurry isconverted to insoluble Al(OH)₃ as the pH is increased. The insolublealuminum hydroxide precipitates onto the fiber. Thus, the resultantfibers are coated with Al(OH)₃ or contain the insoluble metal within thefiber interior. The citric acid sprayed on the web containing the fibersdries on the fibers. When the Al(OH)₃ treated fibers are formed into anabsorbent product, the citric acid creates a locally acidic environmentwhen the citric acid-treated fibers of the absorbent product are exposedto a liquid insult (e.g., urine). The decreased pH created by the acidenvironment converts the Al(OH)₃ to the soluble form of aluminumincluding a citric acid complex of this metal. In this way, aluminumions may become available in solution to locally and temporarily inhibitthe swelling of superabsorbent polymers (also present in the absorbentmaterial) thereby minimizing or preventing gel-blocking.

[0062] In another preferred embodiment, the above procedure is followedto treat the fibers with precipitated Al(OH)₃, and in a subsequent step,aluminum sulfate is applied, preferably by spraying, onto theAl(OH)₃-treated fibers. Preferably the aluminum sulfate is applied tothe web, before the web is introduced to web dryers. Application to thewet web provides better distribution of the aluminum sulfate through theweb. The acidic environment provided by the aluminum sulfate is alsoconducive to release of soluble aluminum ions from the Al(OH)₃precipitate.

[0063] A hierarchy of preferred embodiments is exemplified as follows: atwo component mixture of (1) cellulosic fibers pretreated with a watersoluble aluminum compound and (2) SAP particles in an absorbent core(Example 4), provides a higher level of core permeability than acomparable three component mixture of (1) cellulosic fibers and (2) awater soluble aluminum compound and (3) SAP particles in an absorbentcore (Example 12), and a higher level of core permeability than a twocomponent mixture of (1) SAP particles pretreated with a water solublealuminum compound in an aqueous solution and (2) cellulosic fibers in anabsorbent core (Example 15). These results are exemplified in theprocedures set forth below.

[0064] Treatment of SAP Particles

[0065] Improved core permeability may be obtained by coating the surfaceof SAP particles with a polyvalent ion salt, and combining the coatedSAP particle with a fiber in an absorbent structure. The particles arecoated in contrast to reacting or complexing the SAP particles with apolyvalent cation salt. Coating of the SAP particle with the salt isaccomplished by mixing the SAP particles with a non-aqueous solution ofthe polyvalent ion salt, and subsequently removing the non-aqueoussolvent, leaving a coating of the salt on the surface of the SAPparticle. For example, an anhydrous methanol solution of aluminumsulfate may be mixed with SAP particles at room temperature, for exampleFavor™ SXM 9100, the mixture dried, and the granular coated SAPparticles mixed with fluff fiber in an absorbent core. The corepermeability for such a structure is much higher than that obtained whenan equivalent amount of polyvalent ion salt in aqueous solution is usedto treat SAP particles, indicating superior core permeability withaluminum sulfate-coated particles compared to aluminum cation-complexedSAP particles. Although methanol is the preferred non-aqueous solvent,any solvent which dissolves the salt but does not swell the SAPparticle, may be used. Examples include alcohols, such as ethanol,n-propanol, iso-propanol and acetone.

[0066] The following procedures are employed in the Examples set forthat the end of the specification.

[0067] Formation of Air Laid Structures

[0068] A Kamas mill (Kamas Industri AB, Sweden) is used to disintegratepulp sheets into fluff pulp. A pad former (Buckeye Technologies,Memphis, Tenn.) is used to combine the fluff and SAP particles.

[0069] Laboratory air-laid absorbent structures are made by combiningfiber and SAP particles in the laboratory to simulate the process of anabsorbent core construction on a full-scale commercial line. Fiber andSAP particles are loaded into the pad former. Fiber and SAP particlesare combined through air vortices and become one single structure viathe applied vacuum. The air-laid structure is then die-cut to dimensionsspecific for performance testing. For testing purposes, the airlaidstructure should have dimensions of 14″×14″ at a target basis weight(0.30 g/in² or 0.22 g/in²).

[0070] Measurement of Ion Content

[0071] Metal ion content, including aluminum or iron content, in pulpsamples is determined by wet ashing (oxidizing) the sample with nitricand perchloric acids in a digestion apparatus. A blank is oxidized andcarried through the same steps as the sample. The sample is thenanalyzed using an inductively coupled plasma spectrophotometer (“ICP”)(e.g., a Perkin-Elmer ICP 6500). From the analysis, the ion content inthe sample can be determined in parts per million. The polyvalent cationcontent should be between 0.25% and 5.0% by weight of fibers, preferablybetween 0.25% and 2.5% by weight of fibers, and more preferably between0.4% and 1.2% by weight of fibers.

[0072] Measurement of Ion Extraction

[0073] The percentage of ions extracted from fibers in a saline solutionis measured by submerging the test fibers in a saline solution that isshaken for 24 hours. During this period, ions are extracted from thefibers and into the solution. The ion concentration in the solution ismeasured using an ICP and compared with the ion content in the originalfiber sample to determine the percentage of ion removed due to prolongedexposure to saline under agitation. The ion extraction should exceed 5%,preferably exceed 25%, more preferably exceed 50%, and most preferablyexceed 90%.

[0074] Measurement of Vertical Permeability

[0075] Vertical Permeability is determined using the followingprocedure. This procedure was adapted from the method disclosed in U.S.Pat. No. 5,562,642.

[0076] A Kamas Cell Mill (Kamas Industri AB, Sweden) apparatus is usedto form disintegrated pulp sheets that in turn are used to producefluff. A pad former (Buckeye Technologies Inc., Memphis, Tenn.) is usedto combine SAP particles and fiber to prepare 14″×14″ test pads. Testpads are constructed at a basis weight of 0.3 g/in² and pressed to adensity of 0.15 g/cc. Samples are die-cut to 2¼″ diameter circles andconditioned before testing. The circles are dried in a forced air oven,then placed in a dessicator until the permeability test is run. Thesample is then positioned into a vertical cylinder that contains a base(sample platform) constructed from wire mesh. See FIG. 3 for anillustration of the vertical permeability test apparatus. The verticalcylinder has an inside diameter of 2¼″. A weight placed onto the samplesupplies about 0.3 lb/in² of pressure perpendicular to the sample. Thesample is saturated in fluid (0.9% saline) for one hour. After one hour,the vertical cylinder containing the sample is secured over (but not incontact with) a weighing balance. The sample is initially insulted with50 ml of 0.9% saline via a ⅜″ hole centered in the weight. A 25-mlinsult is added for every 25 grams of fluid that transferred to thebalance until the balance reads 100 grams. Fluid transferred by thesample is measured per unit of time to quantify the permeability for agiven sample. Absorption capacity for the samples is also recorded.

[0077] Measurement of Inclined Permeability

[0078] The following procedure is used to measure inclined permeability.This procedure was adapted from the procedure disclosed in U.S. Pat. No.5,147,343. A Kamas Cell Mill (Kamas Industri AB, Sweden) apparatus isused to form disintegrated pulp sheets that in turn are used to producefluff. A pad former was used to combine SAP particles and fibers toprepare 14″×14″ test pads. Test pads are constructed at a basis weightof 0.22 g/in² and pressed to a density of 0.15 g/cc. Permeabilitysamples are die-cut to eleven square inches and conditioned beforetesting. Refer to FIG. 1 for an illustration of the inclinedpermeability test apparatus used in the procedure. Permeability samplesare placed on a Teflon coated block inclined at a 45-degree angle.Attached to this block is a fluid head box connected by ¼″ tubing to avertically adjustable fluid reservoir. The front edge of the sample padis centered onto and secured to the head box. The head box is designedwith three {fraction (3/16)}″ diameter holes that are spaced {fraction(9/16)}″ apart. A top block coated with Teflon, with a congruent45-degree angle, is placed on top of the sample pad. Lubricated pegs areinserted into the bottom block (sample platform) at a 60-degree angle toprevent the top block from slipping while allowing for uniform sampleexpansion after saturation. A 724.4 g weight, along with the weight ofthe top block supplies about 0.3 lbs/in² of pressure perpendicular tothe sample. The fluid (0.9% saline) level is adjusted to produce andmaintain an inverted meniscus. Once saturation occurs, the sample padacts as a siphon by transferring fluid to a tared receiving containeratop a balance located below the end of the sample. Liquid transferredby the sample is measured per unit time to establish a flow rate.Permeability for a given sample is quantified after the flow ratereaches equilibrium. For example, FIG. 2 shows the incline permeabilityat various time intervals for 50% SAP and 50% cellulose fiber mixtures,and 70% SAP and 30% cellulose fiber mixtures. The figure also shows theincreased permeability produced by the invention fiber in a mixture withSAP (Example 3).

[0079] Calculation of Permeability Factor

[0080] The permeability factor is determined by summing the permeabilityin gm/min from the vertical permeability and the inclined permeability.The sum is taken as follows:

Perm Factor=(vertical²+inclined²)^(1/2)

[0081] where “vertical” permeability and “inclined” permeability areexpress as gm/min. The factor is reported as a dimensionless numberalthough the actual dimensions are gm/min.

[0082] Measurement of Horizontal Wicking (Core Utilization)

[0083] Horizontal wicking samples of about 4″×14″ are placed onto alevel platform with bordering grooves to capture “runoff” fluid (0.9%saline). Both laboratory test cores or manufactured diaper cores may beused. For laboratory cores, an acquisition-distribution layer (ADL) froma commercial diaper cut to 3″×7″ is placed on top of the sample wherefluid is introduced. Then a second board is placed on top of the sampleand ADL. The top board contained an insult reservoir with a {fraction(11/2)}″ inside diameter. The insult region, relative to the sample, was5″ centered from the front end or end closest to the insult reservoir.Two 10 lb. weights placed on the top board along with the weight of thetop board supplied about 0.40 lbs/in² of pressure perpendicular to thesample. Three 100 ml insults were introduced to the sample intwenty-minute intervals. After one hour, the sample was sectioned andweighed to determine the distance that liquid was transported away formthe insult region. Horizontal wicking was quantified by the sum of thelast three inches, on a gram of fluid per gram of core sample basis.

[0084] The following examples are intended to illustrate the inventionwithout limiting its scope.

COMPARATIVE EXAMPLE 1

[0085] A slurry of bleached southern softwood Kraft (BSSK) fibers fromBuckeye Technologies consisting of 4.5 parts fiber/100 parts slurry wasdiluted with sufficient water to provide 0.9 parts fiber/100 partsslurry and adjusted to a pH of 5.5. The resultant slurry wascontinuously dewatered on a sheeting machine where a sheet was formed atrush/drag ratio of 1.0, couched, then pressed and densified throughthree stages of pressing to 48 parts fiber/100 parts slurry. The sheetwas dried using conventional drum dryers to 93.5 percent solids. Thesheet was reeled on a continuous roll.

[0086] Sheets from the roll were defiberized in a Kamas mill. An ionextraction test was performed on the fibers as described above. Theionic extraction of the fiber was measured at 0%. Vertical and inclinedpermeability tests were performed as described above using test coresthat were a mixture of 70% by weight of SAP particles and 30% by weightof fibers. The permeability factor was then calculated. When FAVOR™ SXM70 SAP (obtained from Stockhausen, Inc.) was used, a permeability factorof 16 was obtained.

COMPARATIVE EXAMPLE 2

[0087] Comparative Example 1 was repeated, except that SAP FAVOR™ SXM9100 was used instead of FAVOR™ SXM 70. The permeability factor obtainedwas 141.

EXAMPLE 1

[0088] Cellulose fibers were treated as follows. A total of 9.36 partshydrated aluminum sulfate (Al₂(SO₄)₃*14 H₂O) from General ChemicalCorporation, per 100 parts bleached southern softwood Kraft (BSSK)fibers from Buckeye Technologies were added to a slurry consisting 4.5parts fiber/100 parts slurry. The slurry had a pH of 3.2. After 25minutes of mixing 3.0 parts sodium hydroxide/100 parts fiber were addedalong with sufficient water to provide 0.9 parts fiber/100 parts slurryat a pH of 5.7. The temperature was adjusted to 60° C. The resultantslurry was continuously dewatered on a sheeting machine where the sheetwas formed at 1.0 rush/drag ratio, couched, then pressed and densifiedusing three stages of pressing to 48 parts fiber/100 parts total. Thesheet was dried using conventional drum dryers to 93.5 percent solids.While continuously reeling, a spray of 50% citric acid solution wasapplied to one surface of the sheet at a loading of 2.5 parts per 100parts of fiber. The reeled sheet was then sized into individual rolls.

[0089] The sheet was defiberized in a Kamas mill, and the ionicextraction test described above was performed. The fiber was found tohave an ionic extraction of 34% and an aluminum content of approximately7,500 ppm. Vertical and inclined permeability tests were performed ontest cores using a mixture of 70% by weight of SAP particles and 30% byweight of fibers. The permeability factor using FAVOR™ SXM 70 SAP was31.

EXAMPLE 2

[0090] Example 1 was repeated except that the SAP used was FAVOR™ SXM9100. The permeability factor obtained was 177.

EXAMPLE 3

[0091] A slurry of bleached southern softwood Kraft (BSSK) fibers fromBuckeye Technologies consisting of 4.5 parts fiber/100 parts slurry wasdiluted with sufficient water to provide 0.9 parts fiber/100 partsslurry and adjusted to a pH of 5.5. The resultant slurry wascontinuously dewatered on a sheeting machine and a sheet was formed at arush/drag ratio of 1.0, couched, then pressed and densified throughthree stages of pressing to 48 parts fiber/100 parts slurry. The sheetwas dried using conventional drum dryers to 93.5 percent solids. Thesheet was then reeled. During reeling, 6.1 parts of hydrated aluminumsulfate (Al₂(SO₄)₃*14 H₂O, 50% aqueous solution) is applied by sprayingper 100 parts fiber. The fiber was reeled on a continuous roll. Theresultant reel was sized into individual rolls. The sheets weredefiberized in a Kamas mill and the ionic extraction measured, anddetermined to be 86%. The aluminum content of the fibers was 5,500 ppm.Permeability tests were conducted as described above using test coresthat were a mixture of 70% by weight SAP and 30% by weight fibers. Thepermeability factor using FAVOR™ SXM 70 SAP was 44.

EXAMPLE 4

[0092] Example 3 was repeated except that the aluminum content of thefibers was 5445 ppm, and the SAP used was FAVOR™ SXM 9100. Thepermeability factor obtained was 212. The ion extraction was 86%.

EXAMPLE 5

[0093] 12.1 g of ferric nitrate (Fe(NO₃)₃) (Fisher Chemical Co.) per 152g bleached southern softwood Kraft (BSSK) fibers from BuckeyeTechnologies were added to a slurry of 4.5 parts fiber/100 parts slurry.The slurry had a pH of 2.76. After mixing and dilution to 0.9 partsfiber/100 parts slurry, 27.1 ml of 10% sodium hydroxide were added toprovide a pH of 5.7. The resultant slurry was dewatered on a dynamichandsheet former (Formette Dynamique Brevet, Centre Technique deL'Industrie, Ateliers de Construction Allimand, Appareil No. 48) and waspressed to 48 parts fiber/100 parts total. The sheet was dried to 93.5percent solids. After drying, 2.5 parts of 50% citric acid solution per100 parts of fiber were applied to the sheet.

[0094] The sample sheet was defiberized in a Kamas mill as describedabove. Permeability was determined on test cores formed as describedabove, that were a mixture of FAVOR™ SXM 9100, at 70% by weight andfiber 30% by weight. The permeability factor was calculated to be 178.

EXAMPLE 6

[0095] 9.36 parts hydrated aluminum sulfate (Al₂(SO₄)₃*14 H₂O) per 100parts bleached southern softwood Kraft (BSSK) fibers from BuckeyeTechnologies were added to a slurry consisting of 4.5 parts fiber/100parts slurry. After addition of the aluminum sulfate, the slurry had apH of 3.2. After 25 minutes of mixing, 3.0 parts sodium hydroxide/100parts fiber were added along with sufficient water to provide 0.9 partsfiber/100 parts slurry at a pH of 5.7 and temperature of 60° C. Theresultant slurry was continuously dewatered on a sheeting machine and asheet formed at a 1.0 rush/drag ratio, couched, then pressed anddensified using three stages of pressing to 48 parts fiber/100 partstotal. The sheet was dried to 93.5 percent solids. To this sheet samplewas applied three parts 1,2,3,4-butanetetracarboxylic acid (BTCA) fromAldrich Chemical Company per 100 parts of fiber by spraying a solution.

[0096] The sheet was defiberized in a Kamas mill and the fiber wasdetermined to have an ionic extraction of 12.4%. All permeability factortesting was performed using pads made with 70% by weight of FAVOR™ SXM70 SAP and 30% weight of fiber. The permeability factor was determinedto be 38.

EXAMPLE 7

[0097] 9.36 parts hydrated aluminum sulfate (Al₂(SO₄)₃*14 H₂O) per 100parts bleached southern softwood Kraft (BSSK) fibers from BuckeyeTechnologies were added to a slurry consisting of 4.5 parts fiber/100parts slurry. After addition of the aluminum sulfate, the slurry had apH of 3.2. After 25 minutes of mixing, 3.0 parts sodium hydroxide/100parts fiber were added along with sufficient water to provide 0.9 partsfiber/100 parts slurry at a pH of 5.7 and temperature of 60° C. Theresultant slurry was continuously dewatered on a sheeting machine and asheet formed at a 1.0 rush/drag ratio, couched, then pressed anddensified using three stages of pressing to 48 parts fiber/100 partstotal. The sheet was dried to 93.5 percent solids. To this sheet samplewas applied one part para-toluenesulfonic acid (PTSA) from AldrichChemical Company by spraying per 100 parts of fiber.

[0098] The sheet was defiberized in a Kamas mill and the fiber wasdetermined to have an ionic extraction of 13.4%. All permeability factortesting was performed using test cores made with 70% by weight of FAVOR™SXM 70 SAP and 30% by weight of fiber. The permeability factor wasdetermined to be 32.

EXAMPLE 8

[0099] High porosity commercial fiber (HPZ) was obtained from BuckeyeTechnologies Inc. in sheet form. The fibers had a WRV of 78.7, a curl of51% and a 96.5% alpha cellulose content. A total of 7.7 parts ofhydrated aluminum sulfate octadecahydrate (Aldrich Chemical Company) per100 parts fiber were applied to the sheeted material by spraying.

[0100] Ion extraction was measured for the fiber as 100%. Permeabilitywas measured after preparing a test pad that was 30% by weight of fibersand 70% by weight of FAVOR™ SXM 9100 SAP. The permeability factor was241.

EXAMPLE 9

[0101] High purity commercial cotton fiber (GR702) was obtained fromBuckeye Technologies Inc. in sheet form. A total of 7.7 parts ofaluminum sulfate octadecahydrate per 100 parts fiber were applied to thesheeted material by spraying. Ion extraction was measured for the fiberas 99.0%. Permeability was measured after preparing a pad that was 30%by weight of fibers and 70% by weight of FAVOR™ SXM 9100 SAP. Thepermeability factor was 219.

EXAMPLE 10

[0102] Fibers were prepared as disclosed in U.S. Pat. No. 5,190,563 byapplying 4.7% citric acid and 1.6% sodium hypophosphite to a SouthernSoftwood Kraft pulp sheet. After individualizing and curing at 340° F.for 7.5 minutes, the pulp had a WRV of 44 and a curl of about 75%. Theindividualized fibers were treated by spraying 3.42 parts of hydratedaluminum sulfate (Al₂(SO₄)₃*14 H₂O) per 100 parts fiber were added tothe fibers and the fibers allowed to dry. The ionic extraction for thefibers was measured at 49.8%. The aluminum content of the fibers wasmeasured at 10,869 ppm. Test pads were made with 30% by weight of thetreated fibers and 70% by weight FAVOR™ SXM 9100 SAP and thepermeability factor measured. The factor was found to be 231.

EXAMPLE 11

[0103] A sheet of synthetic hydrophilic non-woven material from BBAcorporation, product number H018B7W, was selected and treated with 1.03grams of aluminum sulfate octadecahydrate per square foot of material byspraying and allowed to dry. Test pads were made from 30% by weightbleached southern softwood Kraft (BSSK) fibers from Buckeye Technologiesand 70% by weight FAVOR™ SXM 9100 SAP, with the treated non-wovenmaterial as a topsheet, and the permeability factor measured. Thepermeability factor was 191.

EXAMPLE 12

[0104] An absorbent core of improved permeability was prepared by adding2.4 parts of aluminum sulfate octadecahydrate (51.3% aluminum sulfate)in powder form to 100 parts of a 30% by weight fiber and 70% by weightSAP core as described in the method for producing cores. Thepermeability factor with FAVOR™ SXM 9100 at 70% SAP was 207.

EXAMPLE 13

[0105] A slurry of bleached southern softwood Kraft (BSSK) fibersBuckeye Technologies consisting of 4.5 parts fiber/100 parts slurry wasdiluted with sufficient water to provide 0.9 parts fiber/100 partsslurry and adjusted to a pH of 5.5. The resultant slurry wascontinuously dewatered on a sheeting machine where the sheet was formedat a rush/drag ratio of 1.0, couched, then treated by spraying with12.35 parts of hydrated aluminum sulfate and 3.17 parts of sodiumhypophosphite per one hundred parts of fiber, then pressed and densifiedthrough three stages of pressing to 48 parts fiber/100 parts slurry. Thesheet was dried using conventional drum dryers to 93.5 percent solids.The fiber was reeled on a continuous roll. The resultant reel was sizedinto individual rolls.

[0106] The sheets were defiberized in a Kamas mill and the ionicextraction of the fiber was measured at 95%. The permeability factor wasdetermined to be 216, using at test core that was 30% by weight fiberand 70% by weight FAVOR™ SXM 9100.

EXAMPLE 14

[0107] A total of 9.36 parts of hydrated aluminum sulfate (Al₂(SO₄)₃*14H₂O) per 100 parts of bleached southern softwood Kraft (BSSK) fibersfrom Buckeye Technologies were added to a slurry consisting of 4.5 partsfiber/100 parts slurry. The slurry had a pH of 3.2. After 25 minutes ofmixing, 3.0 parts of sodium hydroxide per 100 parts of fiber were addedwith sufficient water to provide 0.9 parts fiber per 100 parts slurry ata pH of 5.7 and at a temperature of 60° C. The resultant slurry wascontinuously dewatered on a sheeting machine where the sheet was formedat a rush/drag ratio of 1.0, couched, then treated by spraying with12.35 parts of hydrated aluminum sulfate and 3.17 parts of sodiumhypophosphite per one hundred parts of fiber, then pressed and densifiedthrough three stages of pressing to 48 parts fiber/100 parts slurry. Thesheet was dried using conventional drum dryers to 93.5 percent solids.The fiber was reeled on a continuous roll. The resultant reel was sizedinto individual rolls.

[0108] The sheets were defiberized in a Kamas mill and the ionicextraction of the fiber was measured at 38.2% and the aluminum contentwas 9475 ppm. The permeability factor was determined to be 213, using atest core that was 30% by weight fiber and 70% by weight FAVOR™ SXM9100.

EXAMPLE 15

[0109] An absorbent core was prepared by combining three parts ofdefiberized fluff fiber by weight with seven parts by weight ofpretreated FAVOR™ SXM 9100 SAP. The FAVOR™ SXM 9100 SAP had beenpretreated with aqueous aluminum sulfate octadecahydrate at ratio of 3.7parts of dry aluminum sulfate octadecahydrate to 100 parts of SAP, driedat 125° C. for 3 hours, crushed and sieved to the same particle size asthe untreated SAP. The permeability factor for this core was determinedto be 187.

EXAMPLE 16

[0110] An absorbent core was prepared by combining three parts ofdefiberized fluff fiber by weight with seven parts by weight ofpretreated FAVOR™ SXM 9100 SAP. The FAVOR™ SXM 9100 SAP had beenpretreated with a methanol solution of aluminum sulfate octadecahydrateat a ratio of 3.7 parts of dry aluminum sulfate octadecahydrate to 100parts of SAP, air dried in an exhaust hood to remove visible liquid, andoven dried at 40° C. for two hours. The permeability factor for thiscore was determined to be 268.

What is claimed is:
 1. An absorbent structure, comprising: fibers boundwith a polyvalent cation-containing compound, said fibers exhibiting anion extraction factor of at least 5%; and superabsorbent polymerparticles.
 2. The structure of claim 1 wherein said fibers exhibit anion extraction factor of at least 25%
 3. The structure of claim 1wherein said fibers exhibit an ion extraction factor of at least 50% 4.The structure of claim 1 wherein said fibers exhibit an ion extractionfactor of at least 90%
 5. The structure of claim 1 wherein thepolyvalent cation is present in an amount greater than 0.25% to 5%, byweight of the fiber.
 6. The structure of claim 1 wherein the polyvalentcation is present in an amount of between 0.25% and 2.5%, by weight ofthe fiber.
 7. The structure of claim 1 wherein the polyvalent cation ispresent in an amount of between 0.4% and 1.2%, by weight of the fiber.8. The structure of claim 1 wherein the polyvalent cation is atransition metal ion.
 9. The structure of claim 1 wherein the cation isselected from the group consisting of aluminum, iron, tin and mixturesthereof.
 10. The structure of claim 1 wherein the polyvalent cation isin the +3 or +4 oxidation state.
 11. The structure of claim 1 whereinsaid compound is a polyvalent metal salt.
 12. The structure of claim 9wherein said compound is selected from the group consisting ofhydroxides of aluminum, iron and tin, and mixtures thereof.
 13. Thestructure of claim 9 wherein said compound is selected from the groupconsisting of water soluble salts of aluminum, iron and tin, andmixtures thereof.
 14. The structure of claim 1 wherein said fiber is atleast 80% alpha cellulose and has a water retention value of at least80%.
 15. The structure of claim 14 wherein said fiber is at least 95%alpha cellulose, has a curl of at least 25% and has a water retentionvalue of less than 90%.
 16. The structure of claim 14 wherein said fiberis crosslinked, has a curl of greater than 50% and has a water retentionvalue of less than 60%.
 17. The structure of claim 14 wherein said fiberis a cellulose fiber selected from the group consisting of softwoodcellulose, hardwood cellulose, cotton, esparto grass, bagasse, hemp,flax, chemically modified cellulose, physically modified cellulose,regenerated cellulose, bacterially generated cellulose, lyocell,cellulose acetate and mixtures thereof.
 18. The structure of claim 1wherein said fiber is selected from the group consisting of hydrophobicfibers treated with a surfactant, hydrophobic fibers treated withsilica, surface-oxidized hydrophobic fibers, and mixtures thereof. 19.The structure of claim 1 wherein said superabsorbent polymer is selectedfrom the group consisting of starch-acrylate graft co-polymers,polyacrylates, carboxymethylcellulose derivatives and mixtures thereof.20. The structure of claim 1 wherein the fiber and polymer are in theform of a mixture.
 21. The structure of claim 1 wherein said particlesare present in an amount greater than 40% by weight of said fibers andparticles.
 22. An absorbent structure, comprising: an acquisitionstratum; and a storage stratum in fluid communication with theacquisition stratum, said storage stratum including fibers bound with apolyvalent cation-containing compound, said fibers exhibiting an ionextraction factor of at least 5% and superabsorbent polymer particles.23. The structure of claim 22 wherein the polyvalent cation is presentin an amount greater than 0.25% to 5%, by weight of the fiber.
 24. Thestructure of claim 22 wherein the cation is selected from the groupconsisting of aluminum, iron, tin and mixtures thereof.
 25. Thestructure of claim 22 wherein said superabsorbent polymer is selectedfrom the group consisting of starch-acrylate graft co-polymers,polyacrylates, carboxymethylcellulose derivatives and mixtures thereof.26. The structure of claim 22 wherein said particles are present in anamount greater than 40% by weight of said fibers and particles.
 27. Anabsorbent structure, comprising: an acquisition stratum; and a storagestratum in fluid communication with the acquisition stratum, saidstorage stratum including hydrophilic fibers combined with a polyvalentcation-containing compound, and superabsorbent polymer particles. 28.The structure of claim 27 wherein the polyvalent cation is present in anamount greater than 0.25% to 5%, by weight of the fiber.
 29. Thestructure of claim 27 wherein the cation is selected from the groupconsisting of aluminum, iron, tin and mixtures thereof.
 30. Thestructure of claim 27 wherein said superabsorbent polymer is selectedfrom the group consisting of starch-acrylate graft co-polymers,polyacrylates, carboxymethylcellulose derivatives and mixtures thereof.31. The structure of claim 27 wherein said particles are present in anamount greater than 40% by weight of said fibers and particles.
 32. Adisposable absorbent article, comprising: a chassis including a liquidpervious topsheet, and a liquid impervious backsheet; and an absorbentstructure between said topsheet and said backsheet, said absorbentstructure including: an acquisition stratum in fluid communication withsaid topsheet; and a storage stratum in fluid communication with theacquisition stratum, said storage stratum including fibers bound with apolyvalent cation-containing compound, said fibers exhibiting an ionextraction factor of at least 5% and superabsorbent polymer particles.33. The article of claim 32 wherein the polyvalent cation is present inan amount greater than 0.25% to 5%, by weight of the fiber.
 34. Thearticle of claim 32 wherein the cation is selected from the groupconsisting of aluminum, iron, tin and mixtures thereof.
 35. The articleof claim 32 wherein said superabsorbent polymer is selected from thegroup consisting of starch-acrylate graft co-polymers, polyacrylates,carboxymethylcellulose derivatives and mixtures thereof.
 36. The articleof claim 32 wherein said particles are present in an amount greater than0.25% to 5%, by weight of the fiber.
 37. The article of claim 32 whereinsaid article is selected from the group consisting of infant diapers,training pants, adult incontinence briefs, and feminine hygiene pads.38. A disposable absorbent article, comprising: a chassis including aliquid pervious topsheet, and a liquid impervious backsheet; and anabsorbent structure between said topsheet and said backsheet, saidabsorbent structure including: an acquisition stratum in fluidcommunication with said topsheet; and a storage stratum in fluidcommunication with the acquisition stratum, said storage stratumincluding fibers combined with a polyvalent cation-containing compoundand superabsorbent polymer particles.
 39. The article of claim 38wherein the polyvalent cation is present in an amount greater than 0.25%to 5%, by weight of the fiber.
 40. The article of claim 38 wherein thecation is selected from the group consisting of aluminum, iron, tin andmixtures thereof.
 41. The article of claim 38 wherein saidsuperabsorbent polymer is selected from the group consisting ofstarch-acrylate graft co-polymers, polyacrylates, carboxymethylcellulosederivatives and mixtures thereof.
 42. The article of claim 38 whereinsaid particles are present in an amount greater than 0.25% to 5%, byweight of the fiber.
 43. The article of claim 38, wherein said articleis selected from the group consisting of infant diapers, training pants,adult incontinence briefs, and feminine hygiene pads.
 44. A disposableabsorbent article, comprising: a chassis including a liquid pervioustopsheet, and a liquid impervious backsheet; and an absorbent structurebetween said topsheet and said backsheet, said absorbent structureincluding: an acquisition stratum in fluid communication with saidtopsheet, said acquisition stratum including fibers combined with apolyvalent cation-containing compound; and a storage stratum in fluidcommunication with the acquisition stratum, said storage stratumincluding fibers and superabsorbent polymer particles.
 45. The articleof claim 44 wherein the polyvalent cation is present in an amountgreater than 0.25% to 5%, by weight of the fiber in the storage stratum.46. The article of claim 44 wherein the cation is selected from thegroup consisting of aluminum, iron, tin and mixtures thereof.
 47. Thearticle of claim 44 wherein said superabsorbent polymer is selected fromthe group consisting of starch-acrylate graft co-polymers,polyacrylates, carboxymethylcellulose derivatives and mixtures thereof.48. The article of claim 44 wherein said particles are present in anamount greater than 0.25% to 5%, by weight of the fiber in the storagestratum.
 49. The article of claim 44 wherein said article is selectedfrom the group consisting of infant diapers, training pants, adultincontinence briefs, and feminine hygiene pads.
 50. A disposableabsorbent article, comprising: a chassis including a liquid pervioustopsheet, and a liquid impervious backsheet; and an absorbent structurebetween said topsheet and said backsheet, said absorbent structureincluding: an acquisition stratum in fluid communication with saidtopsheet; a distribution stratum in fluid communication with theacquisition stratum, said distribution stratum including fibers combinedwith a polyvalent cation-containing compound; and a storage stratum influid communication with the distribution stratum, said storage stratumincluding fibers and superabsorbent polymer particles.
 51. The articleof claim 50 wherein the polyvalent cation is present in an amountgreater than 0.25% to 5%, by weight of the fiber in the storage stratum.52. The article of claim 50 wherein the cation is selected from thegroup consisting of aluminum, iron, tin and mixtures thereof.
 53. Thearticle of claim 50 wherein said superabsorbent polymer is selected fromthe group consisting of starch-acrylate graft co-polymers,polyacrylates, carboxymethylcellulose derivatives and mixtures thereof.54. The article structure of claim 50 wherein said particles are presentin an amount greater than 0.25% to 5%, by weight of the fiber in thestorage stratum.
 55. The article of claim 50 wherein said article isselected from the group consisting of infant diapers, training pants,adult incontinence briefs, and feminine hygiene pads.
 56. A method ofpreparing an absorbent structure, comprising: adjusting the pH of aslurry of cellulose fibers to between 3.8 and 4.2; introducing aluminumsulfate to said slurry; agitating the fiber slurry and increasing the pHto between 5.5 and 5.9; forming a web from said fibers; applying anionizable acid in an amount of between 0.5% and 5% by weight of thefibers to said web; drying and individualizing the fibers; andintroducing superabsorbent polymer particles to the fibers to form anabsorbent structure.
 57. The method of claim 56 wherein said acid isapplied by a method selected from the group consisting of spraying,painting and foaming.
 58. The method of claim 56 further comprising thestep of applying a reducing agent to the web.
 59. The method of 58wherein said reducing agent is applied after said application of saidacid.
 60. A method of preparing an absorbent structure, comprising:adjusting the pH of a slurry of cellulose fibers to between 3.8 and 4.2;introducing aluminum sulfate to said slurry; agitating the fiber slurryand increasing the pH to between 5.5 and 5.9; forming a web from saidfibers; applying aluminum sulfate in an amount of between 6.2% and 6.8%by weight of fibers to said web; drying and individualizing the fibers;and introducing superabsorbent polymer particles to the fibers to forman absorbent structure.
 61. The method of claim 60 wherein said aluminumsulfate is applied by a method selected from the group consisting ofspraying, painting and foaming.
 62. The method of claim 60 furthercomprising the step of applying a reducing agent to the web.
 63. Themethod of 60 wherein said reducing agent is applied after saidapplication of said aluminum sulfate application.
 64. A method ofpreparing an absorbent structure, comprising: mixing superabsorbentpolymer particles with an aqueous solution of a polyvalent cationcontaining compound; drying said mixture above 100° C. until a drymixture is formed; crushing the dry mixture to form particles; andintroducing said particles into an absorbent structure containingfibers.
 65. A method of preparing an absorbent structure, comprising:forming a slurry of cellulose fibers; forming a web from from saidfibers; applying aluminum sulfate in an amount of between 6.2% and 7.0%by weight of fibers to said web; drying and individualizing the fibers;and introducing superabsorbent polymer particles to the fibers to forman absorbent structure.
 66. The method of claim 65 wherein said aluminumsulfate is applied by a method selected from the group consisting ofspraying, painting and foaming.
 67. The method of claim 65 furthercomprising the step of applying a reducing agent to the web.
 68. Themethod of 67 wherein said reducing agent is applied after saidapplication of said aluminum sulfate application.
 69. The absorbentstructure of claim 1 wherein said fibers form a topsheet.
 70. Aabsorbent structure, comprising: a non-woven material including fibersbound with a polyvalent cation-containing compound, said fibersexhibiting an ion extraction factor of at least 5%; and superabsorbentpolymer particles.
 71. A method of preparing an absorbent structure,comprising: mixing superabsorbent polymer particles with an non-aqueoussolution of a polyvalent cation containing compound; drying said mixtureuntil a dry mixture is formed; and introducing said particles into anabsorbent structure containing fibers.
 72. The method of claim 71,wherein said non-aqueous solution is prepared with a solvent selectedfrom the group consisting of methanol, ethanol, n-propanol,iso-propanol, acetone and mixtures thereof.
 73. The method of claim 71,wherein the drying step is conducted at a temperature of less than 100°C.
 74. The method of claim 71, wherein the drying step is conducted at atemperature of less than 40° C.
 75. Fibers bound with a polyvalentcation-containing compound, said fibers exhibiting an ion extractionfactor of at least 5%.
 76. The fibers of claim 75 wherein said fibersexhibit an ion extraction factor of at least 25%.
 77. The fibers ofclaim 76 wherein said fibers exhibit an ion extraction factor of atleast 50%.
 78. The fibers of claim 77 wherein said fibers exhibit an ionextraction factor of at least 90%.
 79. The fibers of claim 75 whereinthe polyvalent cation is present in an amount greater than 0.25% to 5%,by weight of the fiber.
 80. The fibers of claim 79 wherein thepolyvalent cation is present in an amount of between 0.25% and 2.5%, byweight of the fiber.
 81. The fibers of claim 80 wherein the polyvalentcation is present in an amount of between 0.4% and 1.2%, by weight ofthe fiber.
 82. The fibers of claim 75 wherein the polyvalent cation is atransition metal ion.
 83. The fibers of claim 75 wherein the cation isselected from the group consisting of aluminum, iron, tin and mixturesthereof.
 84. The fibers of claim 75 wherein the polyvalent cation is inthe +3 or +4 oxidation state.
 85. The fibers of claim 75 wherein saidcompound is a polyvalent metal salt.
 86. The fibers of claim 85 whereinsaid compound is selected from the group consisting of hydroxides ofaluminum, iron and tin, and mixtures thereof.
 87. The fibers of claim 85wherein said compound is selected from the group consisting of watersoluble salts of aluminum, iron and tin, and mixtures thereof.
 88. Thefibers of claim 75 wherein said fiber is at least 80% alpha celluloseand has a water retention value of at least 80%.
 89. The fibers of claim75 wherein said fiber is at least 95% alpha cellulose, has a curl of atleast 25% and has a water retention value of less than 90%.
 90. Thefibers of claim 75 wherein said fiber is crosslinked, has a curl ofgreater than 50% and has a water retention value of less than 60%. 91.The fibers of claim 75 wherein said fiber is a cellulose fiber selectedfrom the group consisting of softwood cellulose, hardwood cellulose,cotton, esparto grass, bagasse, hemp, flax, chemically modifiedcellulose, physically modified cellulose, regenerated cellulose,bacterially generated cellulose, lyocell, cellulose acetate and mixturesthereof.
 92. The fibers of claim 75 wherein said fiber is selected fromthe group consisting of hydrophobic fibers treated with a surfactant,hydrophobic fibers treated with silica, surface-oxidized hydrophobicfibers, and mixtures thereof.
 93. An absorbent structure comprising:fibers bound with a polyvalent cation-containing compound, said fibersexhibiting an ion extraction factor of at least 5%; superabsorbentpolymer particles; and oxalic acid bound to the fiber.
 94. An absorbentstructure comprising: (A) fibers, and (B) superabsorbent polymerparticles having surfaces coated with a polyvalent ion salt containing apolyvalent cation and one or more anions.
 95. The structure of claim 94,wherein the polyvalent cation is present in an amount from about 0.25percent to about 5 percent.
 96. The structure of claim 95, wherein thepolyvalent cation is present in an amount from about 0.25 percent toabout 2.5 percent.
 97. The structure of claim 96, wherein the polyvalentcation is present in an amount from about 0.4 percent to about 1.2percent.
 98. The structure of claim 94, wherein the polyvalent cation isa transition metal ion.
 99. The structure of claim 94, wherein thepolyvalent cation is in the +3 or +4 oxidation state.
 100. The structureof claim 94, wherein the polyvalent cation is aluminum, iron, tin, or amixture thereof.
 101. The structure of claim 99, wherein the anion iswater soluble.
 102. The structure of claim 99, wherein the salt ishydroxide.
 103. The structure of claim 99, wherein the anion is sulfate.104. The structure of claim 103, wherein the salt is aluminum sulfate.105. The structure of claim 94, wherein the superabsorbent polymer is astarch-acrylate graft co-polymer, a polyacrylate, acarboxymethylcellulose derivative or a mixture thereof.
 106. Thestructure of claim 105, wherein the superabsorbent polymer is apolyacrylate.